Aluminum-molybdenum-zirconium-tin master alloys

ABSTRACT

The present invention relates to titanium base alloys, and more particularly to aluminum-molybdenum-zirconium-tin master alloys, which are suitable for further alloying into titanium base alloys. The present invention also relates to methods for producing aluminum-molybdenum-zirconium-tin master alloys, which are useful in providing titanium base alloys containing refractory materials of greater homogeneity. In accordance with the present invention, the tin: zirconium ratio is reduced from about 1:2 to about 1:1, thereby lowering the amount of excess zirconium. After the highest melting point tin: zirconium intermetallic phases have been precipitated, there is little or no excess zirconium to precipitate out with aluminum; therefore, all of the aluminum is available to combine with molybdenum to precipitate the target lower melting point intermetallic phases.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. provisional application Ser. No. 61/782,163 which was filed in the United States Patent and Trademark Office on Mar. 14, 2013.

FIELD OF THE INVENTION

The present invention relates to titanium base alloys, and more particularly to aluminum-molybdenum-zirconium-tin master alloys, which are suitable for further alloying into titanium base alloys. The present invention also relates to methods for producing aluminum-molybdenum-zirconium-tin master alloys, which are useful in providing titanium base alloys containing refractory materials of greater homogeneity.

BACKGROUND OF THE INVENTION

In the production of aluminum-molybdenum-zirconium-tin master alloys, numerous molybdenum-rich zones are typically present. Molybdenum-rich zones are segregated regions within a master alloy containing molybdenum concentrations greater than about 80 weight %. At the present time, molybdenum-rich high-density inclusions (HDIs) within titanium alloys are attributable to either cross contamination by pure molybdenum metal, or to unattributed titanium alloy melting practices. Recent investigations have shown that molybdenum-rich areas can form during the single melt thermite master alloy process. Further analysis of the master alloy chemistry and microstructure has led to the hypothesis that the precipitation sequence of the high melting point intermetallic phases plays an important role in the formation mechanism of molybdenum-rich areas.

In order to prevent the precipitation of any molybdenum-rich areas, the aluminum content should not fall below about 22 weight % at any time during the solidification process. It is believed that both the aluminum: molybdenum and tin: zirconium ratios are important parameters to allow for the lower melting point Al₈Mo₃ and/or Al₆(MoTi) phases to be precipitated in preference to the higher melting point Mo and/or Al:Mo₃(Mo3Al) phases.

In current aluminum-molybdenum-zirconium-tin master alloys, the aluminum: molybdenum and tin: zirconium ratios are 1:1 and 1:2 respectively. A review of related binary phase diagrams shows that the higher melting point tin: zirconium intermetallic phases will precipitate first during solidification, resulting in an excess of zirconium. The excess zirconium combines with aluminum to precipitate the next highest intermetallic phases, which lowers the aluminum available for precipitation with molybdenum, resulting in the precipitation of high molybdenum content intermetallic phases.

SUMMARY OF THE INVENTION

The present invention relates to an aluminum-molybdenum-zirconium-tin master alloy.

In accordance with the present invention, the tin: zirconium ratio is reduced from about 1:2 to about 1:1, thereby lowering the amount of excess zirconium. After the highest melting point tin: zirconium intermetallic phases have been precipitated, there is little or no excess zirconium to precipitate out with aluminum. Therefore, all of the aluminum is available to combine with molybdenum to precipitate the target lower melting point intermetallic phases.

This solidification mechanism may be applied to other titanium master alloys which can potentially suffer from the precipitation of high melting point intermetallic phases in other multi-component master alloys. This mechanism may also be applicable to other critical titanium alloys containing alternative elements, such as niobium and tantalum.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Titanium alloys are commonly produced by melting a blend of pure titanium and a combination of either pure elements and/or master alloys at about 1670° C. Any elements and/or intermetallic phases with a melting point above about 1670° C. may not fully dissolve into the titanium during the repeated melting operations. In addition, elements and/or intermetallic phases with a density above that of titanium will also have a reduced dissolution rate, as they migrate quickly to the solidification zone near the metal-liquid interface. Pure molybdenum and some high molybdenum-containing intermetallic phases, such as Mo₃Al melt above 1670° C. and have higher densities compared to pure titanium, and are therefore considered to be high-density inclusion (HDI) defects. The large (about 300-500 um) molybdenum-rich areas observed in the standard aluminum-molybdenum-zirconium-tin master alloy may not have enough time to dissolve into the titanium during the titanium alloy melting stage. Such areas may also agglomerate into larger molybdenum-rich clusters during the alloy electrode blending and pressing stage prior to melting. By reducing molybdenum-rich areas in the master alloy, the likelihood of producing further molybdenum-based HDI defects in the final alloy is significantly reduced.

It was previously believed that either cross contamination or an inadequate melting process contributed to the formation of molybdenum-rich HDI defects in the final Ti-6Al-2Sn-4Zr-6Mo titanium alloys (Ti-6246 alloys). Details related to such alloys are found in U.S. Pat. Nos. 4,104,059 and 4,119,457, herein incorporated by reference. In accordance with the present invention, it has been shown that both the aluminum: molybdenum and tin: zirconium ratios are key variables in the production of high molybdenum-containing master alloys, and that by careful control of the initial thermite blend chemistry, it is possible to produce a thermite master alloy without the formation of molybdenum-rich areas.

The 6Al-2Sn-4Zr-6Mo master alloy is produced via the single melt, thermite reaction process. The 6Al-2Sn-4Zr-6Mo master alloy has been thoroughly investigated via scanning electron microscopy coupled with energy dispersive x-ray spectroscopy (SEM-EDS). SEM-EDS evaluations were conducted on metallographic mounts of the 6Al-2Sn-4Zr-6Mo master alloy. Numerous molybdenum rich zones were identified in the center and base of the 6Al-2Sn-4Zr-6Mo master alloy. Molybdenum rich zones are segregated regions within a master alloy containing molybdenum concentrations greater than 80 weight %.

The molybdenum rich zones were quantified by increasing the magnification, such that the only region viewed by the SEM-EDS system was the molybdenum rich zone. The typical composition of the molybdenum rich zones characterized in the 6Al-2Sn-4Zr-6Mo master alloy is presented in Table 1, below, prepared by a EVEX SEM-EDS software package.

TABLE 1 Typical Molybdenum Rich Zone Composition in the 6Al—2Sn—4Zr—6Mo Master Alloy Elemental composition in the range of: Elements: WT % AT % AlK 7.39 21.73 ZrL 3.15 2.74 MoL 87.62 72.48 TiK 1.84 3.05

A designed removal of the molybdenum-rich zones was considered by studying and subsequently interpolating relevant phase diagrams with respect to a master alloy containing aluminum, molybdenum, tin, and zirconium. The designed removal was based on selection of a final alloy chemistry that would prevent molybdenum zone enrichment during ingot solidification. In a preferred embodiment, the composition found to produce a master alloy free of molybdenum-rich zones while containing the elemental units aluminum, molybdenum, tin, and zirconium is about 36 weight (wt.)% aluminum, about 36 wt. % molybdenum, about 12 wt. % zirconium, about 12 wt. % tin, and about 4 wt. % titanium. A master alloy of this composition is defined as the 6Al-2Zr-2Sn-6Mo master alloy. Table 2, below, provides a comparison of the 6Al-2Sn-4Zr-6Mo master alloy composition to the composition of the newly designed single melt, master alloy, 6Al-2Zr-2Sn-6Mo. Two Examples were produced in order to target the composition of the RA 6Al-2Sn-4Zr-6Mo master alloy defined in Table 2.

TABLE 2 Master Alloy Composition Comparison 6Al—2Zr—2Sn—6Mo Ti—6Al—2Sn—4Zr—6Mo Element Master Alloy Master Alloy Aluminum 36 wt. % 28.5-31 wt. % Molybdenum 36 wt. % 33-35.5 wt. % Tin 12 wt. % 9-11.5 wt. % Titanium  4 wt. % 3-4 wt. % Zirconium 12 wt. % 19.5-22 wt. %

Example 1

Example 1 was produced using a single melt, thermite reaction; further details of this reaction are found in U.S. Pat. No. 5,769,922, herein incorporated by reference. The composition of this ingot was determined by analyzing samples extracted from the ingot via atomic emission spectroscopy. The elemental composition of Example 1 is found in Table 3, below.

TABLE 3 Example 1 Chemistry Aluminum Molybdenum Tin Titanium Zirconium 33.2 wt. % 36.58 wt. % 10.9 wt. % 3.9 wt. % 14.67 wt. %

A metallographic mount of a bottom—center ingot sample was evaluated via SEM-EDS. While examining this sample at 200 times magnification, molybdenum-rich zones were identified.

The molybdenum rich zones (bright green spots) were quantified by increasing the magnification such that the only region viewed by the SEM-EDS system was the molybdenum rich zone. The molybdenum-rich zone composition is presented in Table 4, below, also prepared by the EVEX SEM-EDS software package.

TABLE 4 Typical Molybdenum Rich Zone Compositionin Example 1 Elements: WT % AT % AlK 6.95 20.55 MoL 90.54 75.26 TiK 2.51 4.18 Mo in excess of about 90%

The number and size of the molybdenum rich zones in this sample were significantly reduced when compared to the 6Al-2Sn-4Zr-6Mo master alloy.

Example 2

Example 2 was produced using a single melt, thermite reaction. The composition of this ingot was determined by analyzing strategic samples extracted from the ingot via atomic emission spectroscopy. Elemental composition of Example 2 is presented in Table 5, below.

TABLE 5 Example 2 Chemistry Aluminum Molybdenum Tin Titanium Zirconium 35.7 wt. % 33.8 wt. % 11.8 wt. % 3.8 wt. % 14 wt. %

A metallographic mount of a bottom—center ingot sample was evaluated via SEM-EDS. After examining this mounted sample at various magnifications, molybdenum rich zones were not identified. Evaluations of the bright green bands displayed indicated that these bands are not molybdenum-rich zones. The typical composition of these bands is presented in Table 6, below, also prepared by the EVEX SEM-EDS software package.

TABLE 6 Typical Composition of Bright Green Bands Elements: WT % AT % AlK 31.38 61.37 ZrL 6.64 3.84 MoL 57.43 31.59 SnL 2.78 1.24 TiK 1.77 1.96

In a preferred embodiment, the maximum ingot weight should not exceed about 120 pounds, with a weight of about 100 pounds being particularly preferred. In a further preferred embodiment, a single stage thermite reaction in a water-cooled copper retort is used.

Thus, the resulting Ti-6246 alloy is an alpha-beta alloy capable of being heat treated to higher strengths in greater section sizes than a Ti-6Al-4V alloy. The properties of this alloy are influenced by its thermo-mechanical history. Enhanced strength, ductility, and low-cycle fatigue properties are contained in the alpha-beta forged material. Beta-forged material contains the best combination of good low-cycle fatigue and fatigue-crack growth resistance. The Ti-6246 alloy is similar in forgeability and crack sensitivity to Ti-6-4. This alloy is used in intermediate compressor stages of turbine engines for disks and blades, seals, and for airframe parts. The Ti-6246 alloy is produced by melting pure titanium with the 33Al-33Mo-11Sn-22Zr-3Ti (6246 MA) Master Alloy. It is known in the titanium aerospace industry that high density inclusion (HDI) defects such as molybdenum-rich inclusions can significantly decrease the mechanical properties of the Ti-6246 alloy, and several molybdenum-rich HDIs have been reported.

In accordance with the present invention, it is possible to produce molybdenum rich zones during the production of the 6Al-2Sn-4Zr-6Mo master alloy. It is believed that the sequence of intermetallic phase precipitations enhances the molybdenum concentration during solidification. The novel 6Al-2Zr-2Sn-6Mo master alloy appears to be free from molybdenum rich zones as a result of careful selection of the final alloy chemistry. This careful selection of chemistry appears to prevent the formation of any molybdenum enrichment zones during ingot solidification.

While the present invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims and this invention generally should be construed to cover all such obvious forms and modifications, which are within the true spirit and scope of the present invention. 

What is claimed is:
 1. An aluminum-molybdenum-zirconium-tin master alloy composition formed by a single stage thermite reaction, said composition comprising about 36 weight % aluminum, 36 weight % molybdenum, about 12 weight % zirconium, and about 12 weight % tin, wherein the alloy is in a form of an ingot.
 2. The composition as recited in claim 1, wherein the ingot has a weight of about 120 pounds.
 3. The composition as recited in claim 2, wherein the ingot has a weight of about 100 pounds.
 4. The composition as recited in claim 1, wherein the alloy is used for disks, blades and seals of turbine engines.
 5. The composition as recited in claim 1, wherein the alloy is used for airframe parts.
 6. The composition as recited in claim 1, wherein the composition is produced by melting pure titanium with the alloy.
 7. The composition as recited in claim 1, further comprising about 4 weight % titanium. 